1. Field of the Invention
This invention relates to viscosity index improver containing oil compositions, particularly lubricating oil compositions, exhibiting reduced haze, and to a process for preparing such compositions. In particular, this invention is directed to low-haze or substantially haze-free lubricating oil compositions and additive packages used in their formulation containing hydrocarbon polymer viscosity index improvers and a haze-reducing effective amount of an oil-soluble hydrocarbyl substituted succinic acid.
2. Description of the Prior Art
An important property of a lubricating composition is the rate at which its viscosity changes as a function of temperature. The relationship between the viscosity and temperature is commonly expressed as the viscosity index (V.I.) Lubricant compositions which change little in viscosity with variations in temperature have a greater viscosity index than do compositions whose viscosity is materially affected by changes in temperature. One of the major requirements of the lubricating oils is a satisfactory viscosity-temperature characteristic so that the oils will not lose their fluidity but will show an equally good performance within a relatively wide temperature range to which they may be exposed in service.
In addition to refining natural petroleum oils to improve their viscosity index characteristics, it has been common practice to introduce long chain hydrocarbon compounds such as linear polymers in order to raise the viscosity index of lubricant compositions. Among the V.I. improvers that have been described in the patent literature are polyisobutylenes as taught in U.S. Pat. Nos. 2,084,501 and 2,779,753; polyalkylmethacrylates as described in U.S. Pat. No. 3,607,749; copolymers of alkylmethacrylates and styrene as shown in U.S. Pat. No. 3,775,329; hydrogenated butadienestyrene copolymers as shown in U.S. Pat. No. 2,798,853; and copolymers of butadiene, styrene and isoprene as shown in U.S. Pat. No. 3,795,615.
It is known to utilize ethylene-alpha-olefin copolymers such as ethylene-propylene copolymers as viscosity index improvers. Thus, lubricants containing copolymers of ethylene and propylene having from 60 to 80 mole % of ethylene and viscosity-average molecular weight in the range of 10,000 to 200,000 have been described in U.S. Pat. No. 3,551,336. U.S. Pat. No. 3,522,180 describes a lubricating oil composition containing a viscosity index improver comprising an ethylene-propylene copolymer having an amorphous structure with a number average molecular weight (M.sub.n) of between 10,000 and 40,000, a propylene content of 20 to 70 mole %, and a M.sub.w /M.sub.n of less than about 5 which is said to provide a substantially shear stable blend with improved viscosity index. U.S. Pat. No. 3,598,738 describes a mineral oil composition containing a viscosity index improver of a class of oil-soluble substantially linear ethylene hydrocarbon copolymers containing 25 to 55 wt. % polymerized ethylene units and from about 75 to 45% of a comonomer selected from the group consisting of unsaturated straight chain monoolefins of 3 to 12 carbon atoms, .OMEGA.-phenyl-alkenes of 9 to 10 carbon atoms, norbornenes and unsaturated non-conjugated diolefins of 5 to 8 carbon atoms which results in systems of outstanding shear stability; and British Patent No. 1,205,243 describes the preparation of ethylene-propylene copolymers, obtained by direct synthesis, having a measurable degree of side chain branching and (M.sub.n) of between 40,000 and 136,000.
The patent literature also discusses the mechanical agitation, churning or other mechanical disruption or degradation of polymeric materials, e.g., U.S. Pat. Nos. 2,727,693; 2,776,274; 2,858,299; and 3,503,948. The degradation of the molecular weight of ethylene-propylene copolymers has become useful in order to make various grades of polymers having different molecular weights and different thickening efficiencies in the lubricating oil. Such a degraded olefin polymer has been found to be useful when the precursor higher molecular weight ethylene-propylene copolymer has an ethylene content in the range of 40 to 85%, a degree of crystallinity of from about 1 to 25 wt. %, and a number average molecular weight (M.sub.n) of from 20,000 to 200,000 as taught by U.K. Patent No. 1,397,994.
It is often found during the preparation, processing, and/or storage of these various oil soluble hydrocarbon polymers that a haze develops in their oil concentrates or oil compositions (e.g., final oil formulations containing the hydrocarbon polymers or their concentrates). The source of this haze does not appear to be the same as that haze resulting from incompatibility of the several additives in a lubricating oil additive concentrate or composition (see U.S. Pat. No. 3,897,353 wherein haze resulting from component incompatibility is overcome in a lubricating oil additive concentrate by blending an amorphous ethylene-propylene copolymer with an n-alkyl methacrylate containing polymer having a number average molecular weight between about 30,000 and about 120,000). Rather, it is believed that this haze is due to the presence of a wide variety of catalysts, metal weak acid salts, etc. which are used in or result from the by-product of the polymerization, finishing process, or other steps in the manufacture or finishing of ethylene-containing copolymers or their oil concentrates. Thus, for example, a typical haze producing substance is calcium stearate having a particle size of from about 0.01 microns to about 15 microns, which calcium stearate is used in the finishing process of ethylene-propylene copolymers useful as V.I. improvers. Generally, oil compositions such as oil concentrates containing the ethylene copolymer viscosity index improver also contain a haze forming amount of these metal weak acid salts. These haze forming amounts are generally less than about 1 wt. % based on the total weight of the oil compositions.
This haze problem was addressed in U.S. Pat. 4,069,162 wherein it is disclosed that haze-free oil compositions containing an oil-soluble hydrocarbon polymeric viscosity index improver such as ethylene-propylene copolymer can be provided by adding to or treating said oil compositions with oil-soluble strong acid containing a hydrogen dissociating moiety which has a pK of less than about 2.5.
It has now been surprisingly discovered that the haze of oil compositions containing a hydrocarbon polymer viscosity index improver can be reduced or substantially eliminated by the addition thereto or treatment thereof with a hydrocarbyl substituted succinic acid, a weak acid containing two hydrogen dissociating moieties both of which have pKs above 2.5 (e.g., succinic acid has a pK.sub.1 of 4.16 and a pK.sub.2 of 5.61).